THIS INVENTION relates to a seal arrangement for effecting a seal between a movable rod, shaft or like element and a passage through which said element extends, for example for sealing a shaft with respect to a bore in which the shaft is movable axially, in a pneumatic system.
One of the key factors in the design of pneumatic systems with sliding elements is the design of the sealing system. Over the years a very large number of such systems have been adopted but all have ultimately been a compromise between achieving a high level of seal integrity and a low level of friction. Considerations of space, cost, tolerance to misalignment, wear rate and required manufacturing tolerances of the containing parts have been other important elements.
The increasing move towards the use of non-lubricated air in pneumatic systems has emphasised the problem of achieving low friction, in order to reduce wear and to obtain other advantages associated with low friction.
Known sealing systems have used a range of designs and employed a variety of materials, combinations of materials and lubricants. one of the simplest seal arrangements is the elastomeric `O` ring and this is still widely used for both static and dynamic sealing. Used in the conventional mode the `O` ring is installed so that it is under constant radial compression. This gives good sealing but because of the tolerances involved and the way in which it is loaded pneumatically it tends to suffer from high wear rates and be intolerant to misalignment.
An alternative method of using an `O` ring is to use it in the so-called `floating` mode. In this mode the ring is installed with a nominal interference with the sliding surface but with radial clearance in the groove, receiving the `O` ring, in the member which slides relative to that sliding surface. In this mode the pressure exerted by the ring on the sliding surface is much reduced and hence so is the friction. The problem with this arrangement is that sealing can now only be achieved if the `O` ring is in contact with the groove wall which is exposed to the lower pressure of any differential across the seal. In some designs it is not difficult to ensure that this is so but in many applications it is possible for conditions to arise which lift the seal off that groove wall. This is particularly true during start up conditions when pressure is first applied. Once a flow is established around under the `O` ring there is no pressure force to push it towards the respective groove wall to establish sealing. It is normal, therefore, to limit the axial clearance between the groove and the `O` ring section to a dimension below which the ring becomes essentially bistable. At the opposite extreme it is not possible to use too small an axial clearance between the `O` ring and the groove because in this case, when pressure is first applied during start-up, the pressure forces the ring down into the groove and once flow is established over the top of the ring sealing cannot be restored.
The net result of these limitations is that, to be sure of reliable operation of floating `O` rings, clearances must be held to a tolerance of the order of 0.05 mm. International Standards for `O` ring dimensions only require the section of smaller `O` rings to be within .+-.0.08 mm. Therefore, selection of `O` rings and matching with grooves is necessary to ensure that a floating `O` ring operates satisfactorily. Even with this care, additional attention has to be paid to the amount of grease applied as too much on the side of the ring can create the same effect as too narrow a groove.